refactor: move loading message delay to config

[kevintraver]

This allows loading_message_delay to be overridden

[stevearc]

In general I want to avoid exposing additional options in the config unless they are necessary. The list of options is already quite large, and it makes it hard for users to find what they're looking for. Could you describe the issue you are trying to solve with this, and why this is necessary?

[kevintraver]

I sometimes see the loading indicator flash, and would like to set it to a higher value.

Maybe this can be something thats not in config by default, but still in the docs and can be added manually.

[stevearc]

What is a value that would cause it to not flicker for you? 100ms was pulled from nowhere in particular, and I'm probably fine with any number below 500ms. I wouldn't even blink at bumping to 200ms.

[kevintraver]

Yeah, it does not flicker at 200ms.

[stevearc]

Delay updated to 200ms